The present invention relates to a process for shaping the end of a tube with an oblong cross-section, in particular with a flat section, to a circular cross-section, in which shaping occurs in a single stage of radial compression from the exterior.
Processes of this type are used in the construction of so-called mechanical assembly-type heat exchangers in which an array of tubes having oblong cross-sections (this term is intended to cover flat, oval, ovoid and similar cross-sections) are fixed to a stack of sheets by radial expansion of the tubes. Subsequently, the ends of the tubes are shaped along a circular cross-section and these ends are inserted in corresponding holes in base plate. The tubes are fixed to the base plate by radial expansion which brings the outer wall of each tube into contact with an elastomeric sealing collar.
The rounding process for the end of a tube having a cross-section which is not circular should produce a deformation which is as regular as possible, since the deformed part should be able to come into contact with the sealing element.
In addition to the uniformity of the section obtained by the plastic deformation, it is necessary for the outer surface of the deformed zone to be completely free of scoring and to be perfectly smooth, in order to avoid micro-losses between the tube and the seal even after the exchanger has been operating for several hours.
The present invention relates specifically to a process in which shaping is performed in a single radial compression stage from the exterior. The conventional method of shaping the tube acting exclusively on its outer surface provides for the use of two jaws with a semi-circular profile which act along the longest axis of the cross-section of the tube. The disadvantage of this process is that there is a risk of producing distinct pinched areas on the exterior of the tube or, at best, scoring which is more or less deep. This occurs since, in the absence of an internal guide in the tube during the deformation process (which can be a punch which perfectly matches its internal diameter once performed), it is very difficult to centre the tube on the device and to force the tube to be deformed in a regular and uniform manner along the entire perimeter of the section.
In fact, it is very easy for one wall of the tube to yield before the other and for it to be outside the theoretical deformation profile whilst the jaws close. With a shaping tool having two jaws, in order to exclude the risk of pinching the tube, it would be necessary to increase the diameter of deformation in such a way that the final result would be a section which is rounded only approximately and this section would be so large that it would force the distance between the tubes to be increased.
Furthermore, in a shaping tool having two jaws, the corners deriving from the intersection of the closure plane of the jaws with the deformation hole represent a scoring risk, since they could cut into the outer surface of the tube which is compressed against them by the deformation effect. In fact, given that the action of an inner punch is not utilized, it is necessary to compress the tube to a greater degree on its outer surface in order to obtain a sufficiently uniform circular section.